1. Technical Field
Embodiments of the present invention generally relate to a charging roller, a charging device, and an image forming apparatus, such as a copier, a printer, a facsimile machine, a plotter, or a multifunction peripheral (MFP) including at least two of copying, printing, facsimile transmission, plotting, and scanning capabilities.
2. Description of the Related Art
In electrophotographic image forming apparatuses such as copiers, printers, and facsimile machines, an electrostatic latent image is formed on an image bearer and developed with toner into a visible image. Then, the image is transferred onto a sheet (i.e., a recording medium) and fixed thereon.
Image forming apparatuses employing electrophotography or electrostatic recording typically include a charging device to charge the surface of the image bearer such as a photoconductor. For example, a corona charging device is disposed contactlessly with a target to be charged so that a discharge opening thereof faces the target. The surface of the target is exposed to corona current flowing from the discharge opening, and then the target is charged to a predetermined potential in a predetermined polarity.
Corona charging devices require high-voltage power supplies and generate discharge products such as ozone and nitrogen oxide due to corona discharge, and charging efficiency thereof is relatively low. Further, discharge wire is likely to be soiled.
Currently, progress has been made in contact-type charging devices that are lower in power consumption, higher in charging efficiency, and generate a smaller amount of discharge products. Contact-type charging devices include a conductive charger to be disposed in contact with the target to be charged. Voltage is applied to the charger to induce discharging toward the target so that the surface of the target is charged to the predetermined potential. The charger can be any of a roller, a blade, a rod, and a brush.
It is to be noted that, similar to the arrangement in which the charger is disposed in contact with the target, the target can be charged by applying a charging bias to the charger disposed facing the target across a small clearance that allows discharging between the charger and the target. Since the charger does not contact the target, toner, paper dust, contaminant, toner additives, and the like adhering to the target are less likely to adhere to the charger. This configuration can obviate the necessity of cleaning of the charger or simplify a cleaning structure therefor. Accordingly, the device can be simplified and reduced in size, and damage to the charger due to friction can be inhibited.
Widely used in contact-type charging is roller charging, which uses a conductive roller. Conductive rollers typically include a metal core and an elastic conductive layer (made of rubber, for example) overlying the metal core. Reliable charging is available with roller charging.
Types of bias application of contact-type charging include a direct current (DC) charging, in which DC voltage as the charging bias is applied to the charger, and an alternating current (AC) charging, in which the charging bias includes AC voltage superimposed on DC voltage. In either charging type, the surface of the target is charged to the predetermined potential by the contract-type charger, to which the charging bias is applied.
Uniform discharging is difficult in DC charging, and practical use of DC charging is difficult unless an ion-conductive component is used. Thus, usable materials are limited. Additionally, the resistance value of the ion-conductive component largely depends on environments, and half a digit or double-digit changes in resistance value are possible in response to temperature changes. Additionally, surface irregularity of the roller is likely to result in uneven charging, which can degrade image quality. Even if the surface irregularity is reduced, it is possible that surface properties thereof are degraded due to abrasion over time or toner and dust adhering thereto. Then, uniform charging becomes difficult.
By contrast, AC charging is superior to DC charging in terms of uniform charging potential (tribo-electric potential). Uneven image density is suppressed and streaks in images due to poor charging are less likely to occur in AC charging.
In AC charging, however, the charging currant is larger, and damage to the image bearer such as the photoconductor is larger. Accordingly, compared with DC charging, the image bearer, which is charged, is abraded more over time, and the operational life of the image bearer is reduced. Specifically, when an excessive amount of AC voltage is used, the amount of AC discharge current flowing between the charger and the target increases. As a result, the surface of the photoconductor, which is the target to be charged in the image forming apparatus, is abraded. Thus, degradation of the surface of the target is promoted. Additionally, under hot and humid conditions, the occurrence of image failure such as image deletion, caused by discharge products adhering to the target, increases.